This invention relates to an image forming apparatus used on a copying machine or printer and which is capable of double-sided printing and more particularly, to an intermediate tray portion of an image forming apparatus on which a plurality of copy sheets having been finished with image formation on one side thereof are temporarily stacked in a nipped state for transport of the copy sheets again (refeed of the copy sheets) to conduct image formation on the opposite side thereof.
Conventionally, there has been known copying machines capable of double-sided copying, provided with sheet inverting mechanism including a so-called "switch-back transport path, SB" and intermediate tray portion "IT" (see FIGS. 7A and 7B). In such copying machine, as in FIG. 7A, for the double sided copy, the sheet is inverted after passing through the area of imaging assembly, then the inverted sheet is transferred to the intermediate tray. Thereafter the sheet is moved in the same direction to the imaging assembly to obtain a copy on the side of the copy sheet on which no image formation is completed. Similarly, as in FIG. 7B, for the double sided copy, the sheet is inverted at the switch-back transport path, SB provided upstream of the intermediate tray and the inverted sheet is transferred onto the intermediate tray. Irrespective of the fact that the switch-back transport path "SB" is provided at an upper portion or the lower portion of the image forming apparatus, there is a common feature for these simplified structures of conventional mechanisms that the copy sheet having finished an image formation on one side thereof is inverted before being introduced onto the intermediate tray.
In other words, the sheet inverting mechanism (switch-back transport path) is disposed independently from the intermediate tray portion, i.e., upstream of the intermediate tray portion with respect to sheet transport direction. Thus copy sheets having been finished with an image formation on one side in an imaging assembly are fed to the intermediate tray portion one after another via the sheet inverting mechanism where the sheet transport direction is inverted (i.e., the surface of copy sheet is turned over) so as to execute an image formation on the opposite side of the copy sheets. The copy sheets which are temporarily stacked on the intermediate tray portion after the sheet inverting mechanism are transported again from the intermediate tray portion to the imaging assembly in the same direction that they were transported thereto, upon designation of image formation on the opposite side, thereby executing the image formation on the opposite side in the imaging assembly.
Note that throughout the description, "sheet feed direction" is the direction along which copy sheets are transported until they reach the intermediate tray portion, whereas "sheet refeed direction" is the direction along which they are transported again after being temporarily stacked on the intermediate tray portion.
As mentioned above, in the conventional image forming apparatus, copy sheets stacked on the intermediate tray portion are transported for refeeding in the same direction to the imaging assembly as where they were transported to the intermediate tray portion. In other words, sheets fed to the intermediate tray will not change the direction of movement as they are fed again in the same direction as they were fed to form the image on the other side of the copy sheet to complete the double sided copy.
There has been an increasing demand for image forming apparatus of a smaller size. In producing smaller-sized image forming apparatus, the idea of eliminating the independent switch back transport path is considered to be one of the solutions.
However, there are expected various difficulties to overcome if the above idea is to be adopted such as how the independent switch back transport path is eliminated; where in the sheet refeeding path the switch back transport path is suitably accommodated; which one of the parts in the refeeding path can also function as the switch back transport path.